Exon 3 genotypes of OPN1LW/OPN1MW associated with X-linked congenital cone dysfunction

Abstract

Purpose: To assess exon 3 genotypes of the X-linked OPN1MW/OPN1LW gene cluster encoding the cone photoreceptor red and green opsins, in patients with X-linked blue cone monochromatism (BCM) and congenital cone dysfunction (XLCD). Large deletions affecting the locus control region (LCR), sometimes also extending into the opsin genes or complete gene cluster deletions, genomic rearrangements leading to hybrid gene formation and simultaneous inactivating missense or nonsense mutations are the cause of BCM. Yet in a small percentage of cases, the causative mutation could not be identified. Recent publications have shown that certain combinations of rare sequence variants in exon 3 of the OPN1MW/OPN1LW genes can result in missplicing and exon skipping, leading to XLCD.

Methods: DNA sequencing and genotype evaluation of PCR amplified exon 3 of OPN1MW/OPN1LW in clinically diagnosed BCM or XLCD patients in whom the structure of the OPN1MW/OPN1LW gene cluster had previously been determined by means of PCR and PCR/RFLP analysis.

Results: In our cohort of 187 clinically diagnosed BCM patients, ~80% carry either large genomic deletions (28.8%), the common missense mutation p.C203R (46%), or other inactivating missense or nonsense mutations (5.9%). We here describe the genetic basis of BCM/XLCD of at least 11 patients from 7 independent families carrying genotypes that have recently been acknowledged by the encoded amino acid determinants p.L/M153 - p.V/I171 - p.V/A174 - p.V/I178 - p.A/S180. In 3 families the structure of the OPN1MW/OPN1LW gene cluster was essentially intact, but the proximal gene was shown to carry the deleterious LIAVA (n=2) or LVAVA (n=1) genotype, while the distal gene(s) were either LVAIA, MVVVA or MIAVA. In the remaining 4 families the gene cluster was replaced by a single red- or red-green hybrid gene that carried the deleterious genotypes LIAVA (n=2 families, 4 patients) or LVAVA (n=2 families and patients).

Conclusions: Certain rare sequence variants in exon 3 of the OPN1MW/OPN1LW gene cluster give rise to missplicing and exon skipping. We conclude that these kinds of previously unrecognized splice mutations may account for many of the remaining genetically uncharacterized BCM/XLCD cases, as our first analysis has shown that 5.9% of patients of our total BCM cohort carry these deleterious genotypes.

Keywords: 696 retinal degenerations: hereditary • 470 color pigments and opsins • 539 genetics